1. Field of the Invention
The present invention relates generally to a transmit diversity system, method, and computer program product, which employ Orthogonal Frequency-Division Multiplexing Code-Division Multiplexing (OFDM-CDM) in which two-dimensional (i.e., time and frequency-domain) spreading is performed, and more particularly to a space-time transmit diversity system and a transmit-antenna array used in a mobile communication system.
2. Description of the Related Art
FIG. 13 is a diagram illustrating a space-time transmit diversity technique that uses space-time (ST) codes, which was proposed in a paper by S. M. Alamouti, entitled “A Simple Transmit Diversity Technique For Wireless Communications”, IEEE Journal on Selected Areas In Communications, Vol. 16, No. 8, pp. 1451-1458, October 1998. According to Alamouti's transmit diversity technique, a 2×2 orthogonal space-time code matrix, for two transmit symbols s1, s2, generated by a space-time encoder 1300 illustrated in FIG. 13 is given by Equation 1:
                    Ω        =                  [                                                                      S                  1                                                                              S                  2                                                                                                      -                                      S                    2                    *                                                                                                S                  1                  *                                                              ]                                    (        1        )            
At time t1, a transmission signal s1 is transmitted from a first antenna 1301, and a transmission signal s2 is simultaneously transmitted from a second antenna 1302. At time t2, a transmission signal −s2* is transmitted from the first antenna 1301, and a transmission signal s1*, is transmitted from the second antenna 1302.
Referring to FIG. 13, “h1” denotes a channel response from the first antenna 1301 to a terminal 1303, and “h2” denotes a channel response from the second antenna 1302 to the terminal 1303. Received signals r1 and r2 at times t1 and t2 are expressed by Equations 2 and 3, respectively.r1=h1s1+h2s2  (2)r2=−h1s2*+h2s1*  (3)
A receiver in the terminal 1303 decodes the received signals based on the channel response h1 from the first antenna 1301 and the channel response h2 from the second antenna 1302, and the decoded signals are expressed by Equations 4 and 5, respectively.ŝ1=h1*r1+h2r2*=(|h1|2+|h2)s1  (4)ŝ2=h2*r1−h1r2*=(|h1|2+|h2)s2  (5)
From the decoded signals, it is possible to detect the transmission signals s1 and s2, and also to achieve a maximum ratio combination.
FIGS. 14 to 16 illustrate a transmit diversity technique for improving transmission characteristics through optimal beam selection, which was proposed in a paper by M. Fuji, entitled “Beamspace-Time Transmit Diversity For Time-Domain Spreading OFDM-CDM Systems”, IEICE Trans. on Communications, Vol. E86-b, No. 1, pp.344-351, January 2003.
In the transmit diversity technique using the optimal beam selection, a base station uses fixed multiple beams. However, each mobile station estimates a channel response from each of the multiple beams to calculate power of each beam (for example, add powers of all subcarriers in the multicarrier scheme). Then, the mobile station selects two neighboring beams (or a pair of neighboring beams), which provide the largest channel response power sum, and sends a beam-pair index representing the two selected beams to the base station. In this transmit diversity technique, the transmitter space-time encodes signals for transmission using a 2×2 orthogonal space-time coding matrix, and assigns the space-time coded signals to two beams appointed by the base station. In addition, a signal weighted by a beam forming array weight vector is spread using the OFDM-CDM that performs only the time-domain spreading. The spread signal is then multiplexed with signals of the other users.
Additionally, the receiver performs time domain dispreading to suppress all signals for the other users and thus decode a desired signal.
FIG. 17 illustrates how space-time coded signals are allocated to spread areas in a conventional space-time transmit diversity system. A space-time encoder 1701 space-time encodes data for transmission to output two space-time coded signals [s1, s2] and [−s2*, s1*]. The two space-time coded signals [s1, s2] and [−s2*, s1*] are sequentially beam-steered by beam steering vectors 1702 and 1703, respectively, and are then multiplexed at an adder 1704.
Two signals (s1Wb1+s2Wb2, −s2*Wb1+s1*Wb2) output from the adder 1704, are spread in the time domain (or in the time direction) at two spread areas (2×SFTime). To maintain orthogonality between codes in time-domain despreading, the spreading factor must be limited to the extent that there is no influence of channel variations in the time domain. For the space-time codes, it is required that the channel response be invariant depending on time slot lengths of a number of symbols outputted in the time domain. As a result, the design must be implemented such that there is no influence of variation in the time domain over the two spreading areas. However, if the time-domain spreading factor is limited below a predetermined value, the number of users that can be accommodated is decreased. In addition, if there is an influence of time variation over the two spreading areas, transmission characteristics are worsened.
In the two-beam selection method described above, if the user is located between two neighboring beams as illustrated in FIG. 18, beam diversity gain can be achieved, thereby improving transmission characteristics. If the user is located near the maximum gain of a beam as illustrated in FIG. 19, and the angular spread of a corresponding electromagnetic wave is narrow compared with the width of the beam, the beam gain can be achieved. However, despite the use of the multiple beams, only signals transmitted in substantially one beam reach a mobile station (corresponding to the user), thereby wasting signal power distributed to the other beams.
According to the above-described space-time transmit diversity technique, as signals for transmission are spread only in the time domain, all signals other than a signal for the user are suppressed in the despreading at the receiver. For example, if a number of users use different pairs of beams but one of the two beams for each beam pair is shared by different users, signals for the different users interfere with each other when the space-time codes are decoded. As a result, at a receiver of one user, all signals for the other users are suppressed through time-domain despreading to prevent the interference.
However, according to the two-dimensional spreading, in which all the time domain spreading and the frequency domain spreading are performed, interference between space-time codes allocated to one pair of beams and space-time codes allocated to the other pairs of beams, which share one beam, may not be suppressed in the time-domain partial despreading (at the two-dimensional spreading area) when the space-time codes of said one pair of beams are decoded. For example, when first and second beams #1 and #2 are used for a user #1, and the second beam #2 and a third beam #3 are used for a user #2 as illustrated in FIG. 20, if signals transmitted to the user #1 are denoted by (s1, s2) and signals transmitted to the user #2 are denoted by (s3, s4), channel responses at an m-th subcarrier from the first and second beams #1 and #2 to the user #1 are denoted respectively by hm,1 and hm,2, signals received by the first user #1 at the m-th subcarrier are given by Equations 6 and 7.rm,1=hm,1s1+hm,2s2+hm,2s3  (6)rm,2=−hm,1s2*+hm,2s1*−hm,2s4*  (7)
The received signals rm,1 and rm,2 are decoded using the channel responses hm,1 and hm,2, and the decoded signals can be expressed by Equations 8 and 9.ŝm,1=hm,1*rm,1+hm,2rm,2*=(|hm,1|2+|hm,2|2)s1+hm,1*hm,2s3−|hm,2|2s4  (8)ŝm,2=hm,2*rm,1−hm,1rm,2*=(|hm,1|2+|hm,2|2)s2+|hm,2|2s3+hm,1hm,2*s4  (9)
As shown in second and third components in the right-hand sides of these equations, two-dimensional spreading causes interference between the signals for the users #1 and #2.
Accordingly, for the time-domain spreading method, because the method uses spreading codes in the range where the channel responses can be considered invariant, it is possible to suppress signals of the other users through the despreading, without causing interference. However, according to the two-dimensional spreading method, partial correlation between spreading codes is not necessarily zero at each subcarrier, causing the interference components as described above. Because the decoded components and the response component (|hm,1|2+|hm,2|2) of the user's signal are different, it is impossible to completely remove the interference components even if frequency-domain combination is performed.